1. Field of the Invention
The present invention relates to a method of purifying exhaust gas and an exhaust gas purification system performing a forced regeneration control by accurately judging the regeneration time of a diesel particulate filter even in the case of performing a low rotation speed driving such as driving while the vehicle is stopped and driving while idling for a very long time.
2. Description of the Related Art
The regulation of the exhaust amount of particulate matter (PM: Particulate Matter: below, referred to as PM) exhausted from a diesel engine has been reinforced every year as well as that of NOx, CO, HC, etc. Techniques to decrease the amount of PM exhausted to the outside have been developed by collecting this PM with a filter called a diesel particulate filter (DPF: Diesel Particulate Filter: below, referred to as DPF). Among those, there is a continuous regeneration type DPF apparatus carrying a catalyst.
In this continuous regeneration type DPF apparatus, PM collected by the filter is continuously combusted and purified, and the filter self-regenerates when the exhaust gas temperature is about 350° C. or more. However, in the case that the exhaust temperature is low, because the temperature of the catalyst decreases and does not activate, the oxidation reaction is not promoted. Because of that, it becomes difficult to self-regenerate the filter by oxidizing the PM. As a result, clogging-up progresses due to the deposition of PM onto the filter, and therefore a problem of raising the exhaust pressure is generated due to this clogging-up.
Then, when the clogging-up of the filter exceeds a predetermined amount, a forced regeneration control is performed in which the collected PM is forcibly combusted and removed by forcibly increasing the temperature of the exhaust gas by a multi injection (a multiple stage delay injection), a post injection (after injection), etc. in a cylinder. In this forced regeneration control, HC (hydrocarbon) that is supplied into the exhaust gas by the post injection, etc. is combusted with an oxidation catalyst that is arranged in the upstream side of the filter or an oxidation catalyst that is carried in the filter. Using this combustion heat, the exhaust gas temperature at the filter entrance or the filter surface is increased. PM is combusted and removed by increasing the temperature of the filter to a temperature at which PM that is accumulated in the filter is combusted or more with this exhaust gas that is at a high temperature.
The degree of this clogging-up of the filter is generally judged by the pressure difference between the front and the back sides of the filter. However, in the case of performing idling for a long time while turning an engine on in cold districts where it is difficult to start the engine, in the case of being caught in a traffic jam, in the case of parking a refrigerator car, etc. day and night, etc., it becomes a stopped and idling condition, and the flow rate of the exhaust gas is small. Because of that, it is difficult to estimate the accumulated amount of PM from the pressure difference between the front and the back sides of the filter, and the regeneration time cannot be accurately judged. Therefore, with the judgment only by the pressure difference between the front and the back sides of the filter, the timing to start the forced regeneration control is missed, and a possibility is generated in which PM is over-collected.
As one of the countermeasures of the decrease of accuracy of this pressure difference, as described in Japanese Patent Application Kokai Publication No. 2005-299585 for example, a device for detecting the collected amount of particulates in the exhaust from engine is proposed in which the engine is controlled so that the exhaust flow rate temporarily becomes a predetermined flow rate or more every time the vehicle travels for a predetermined distance or every time the stopped idling time reaches to a predetermined time when the exhaust flow rate is less than the predetermined flow rate and the PM collected amount is estimated from the front-back pressure difference during this time. However, in this device, a control of the engine is performed in which the exhaust flow rate is temporarily increased while driving when the exhaust flow rate is low. Because of that, the driver feels a sense of incongruity, and there is a problem that fuel efficiency deteriorates.
Further, as described in Japanese Patent Application Kokai Publication No. 08-61043, a particulate filter regeneration device is proposed in which a judgment of the regeneration starting time by a particulate amount detection means (the pressure difference between the front and the back sides of the DPF or a constant passage of time) and a judgment of the regeneration starting time by the running time from the previous filter regeneration process are used together, and in the case that any of the judgment results is a time to start, a regeneration means is started. However, in the judgment of the regeneration starting time by constant passage of time and the judgment of the regeneration starting time by the running time from the previous filter regeneration process, various driving conditions are passed through in which the PM accumulated amount per unit time differs during the constant time or during the running time. Because of that, it is difficult to estimate an accurate PM deposition amount, and there is a problem that an error is generated in the regeneration time.
Furthermore, as described in Japanese Patent Application Koai Publication No. 59-28011 for example, an exhaust purification device of a diesel engine is proposed in which the clogging-up condition of the filter member is determined from the sum of two values of which each of the running distance and the idling time is considered. However, in the judgment by the running distance, it is difficult to estimate an accurate PM deposition amount, and there is a problem that an error is generated in the regeneration time because various driving conditions are passed through in which the PM accumulated amount per unit time differs even if it is the same running distance.    Patent Document 1: Japanese Patent Application Kokai Publication No. 2005-299585    Patent Document 2: Japanese Patent Application Kokai Publication No. 08-61043    Patent Document 3: Japanese Patent Application Kokai Publication No. 59-28011